PROJECT ABSTRACT Despite now conclusive evidence that alterations in gut microbial communities precede and contribute to the etiopathogenesis of inflammatory bowel disease (IBD), the promise of therapeutic strategies to favorably influence this ecology is still largely unrealized. In sharp contrast to the widely understood importance of well- characterized taxonomic changes that occur during transitions from health to disease, comparatively little is known about the specific microbially-mediated processes that contribute to this loss of homeostasis. This disparity is largely due to the fact that even in well-studied communities, such as the human gastrointestinal (GI) tract, only a small fraction of the genomic content of a sample, the metagenome, can be functionally annotated. To address this glaring deficiency, we propose to develop a novel computational framework to infer a microbial gene?s metabolic function using quantitative metagenomics and sequence similarity network analysis. We will then apply this method to more comprehensively evaluate the role of sulfur-metabolizing bacteria in IBD. Sulfur- metabolizing bacteria are a phylogenetically diverse group of microbes defined by their ability to process dietary sulfur, often generating hydrogen sulfide (H2S) as a harmful byproduct. H2S in the GI tract can compromise gut barrier integrity by causing a breach in the protective mucus bilayer, a necessary precursor to intestinal inflammation. Our central hypothesis is that higher abundance of sulfur-metabolizing bacteria is associated with greater disease activity, and this community will prove amenable to selective depletion through food avoidance. Our overall objective is to comprehensively identify the bacterial species and strains participating in sulfur metabolism by first cataloging which of them encode known or novel sulfur metabolizing enzymes. We will then determine how these bacteria, their transcriptional activities, and the metabolites they generate influence disease severity in a cohort of densely sampled IBD patients who provided stool at up to 24 timepoints over one year along with short- and long-term assessments of dietary intake. Finally, we will develop and implement a rational dietary avoidance strategy designed to specifically target these bacteria and starve them of the foods that fuel this process, concluding with a randomized controlled trial testing this intervention in IBD patients. The scientific rationale to pursue this line of inquiry is rigorously supported by a body of literature demonstrating that: 1) both diet and the presence of select sulfur-metabolizing bacteria influence IBD severity and 2) preliminary efforts led by the candidate reveal that diet may modulate the relative abundance and functional activities of these bacteria. The approach requires innovative scaling solutions to apply homology-based methods to fully characterize an entire biochemical pathway?microbial sulfur metabolism?in humans. Anticipated outcomes from this multidisciplinary effort include the development of an open-sourced methodological framework for hypothesis- driven microbiome research and the creation of a patient-friendly IBD treatment based on dietary avoidance.